1. Field of the Invention
The present invention relates to an automatic focusing camera.
2. Description of the Prior Art
There have been proposed automatic focusing devices of the type comprising light which has receiving means for receiving light passed a photographic lens and the amount and direction of defocus of the photographic lens relative to a predetermined focal plane of the photographic lens are detected by the output value of the light receiving means , whereby the photographic lens can be moved to the in-focus position by the result of the detection. In such an automatic focusing device, if accuracy of the light receiving means and control accuracy of positioning of the photographic lens are complete, the photographic lens may be positioned at a desired position by the result of one operation of the defocus detection device. However, actually, the accuracy of the defocus detection is not complete, in particular, in a case where the defocus value is large, the error of the defocus detection is large.
Thus, there may be conceived such an arrangement that the photographic lens is moved toward the in-focus position based on the result of the defocus detection of a first time, then the lens is once stopped, to detect the defocus condition again, subsequently the lens is moved toward the in-focus position based on the result of the second detection. By repeating the above operation, the lens can be positioned at the in-focus position where the defocus value is negligible small. This arrangement, however has a drawback in that the lens is repeatedly moved and stopped, and a smooth automatic lens focusing can not be made. In addition in case of employing a CCD sensor array i.e., charge accumlating device as the light receiving means, since it takes a relatively long charge and discharge time and calculation time of the defocus, whereby a rapid automatic focusing can not be expected.
In order to eliminate the drawbacks mentioned above, U.S. Pat. Nos. 4,387,975 and 4,445,761 disclose automatic focusing devices for correcting the error occurred during charge and discharge of the CCD light sensor array by repeating defocus detection causing the lens to be moved toward the in-focus position. However, such correction of the error during movement of the lens can be possible only when the lens speed is kept constant. On the contrary, in the arrangement shown in U.S. patents, the lens speed may be fluctuated at the beginning of the movement of the lens, accordingly a good accuracy of the correction can not be expected. In addition, according to the arrangement, there is required to provide a memory device for monitoring the amount of movement of the lens, and manufacturing cost becomes expensive, if a microcomputer is employed.
Another arrangement disclosed in Japanese Patent Publication of No. 20322/1978 U.S. Pat. No. 4,247,188 comprises a touch switch which becomes conductive by contact of the finger to a release button of a camera, a light measurement switch which is caused to be conductive by pushing the release button down to a predetermined depth and a release switch which is caused to be conductive by pushing the release button down to a further predetermined depth, whereby light measurement calculation and display are executed when the finger touches the touch switch. When the light measurement switch is conductive, only the above operation is continued. When the release switch is conductive, exposure control is started.
In case of application of the automatic focusing device to the above mentioned arrangement, there occurs a problem of what operating member is used for initiating the automatic focusing device. A first possible arrangement is to use another operating member different from the shutter release button for starting the automatic focusing device. This first arrangement must be provided by said another operating member, whereby operability is bad. A second possible arrangement is to start the automatic focusing upon conduction of the touch switch. However, since the touch switch can be conductive when an electric conductive article touches the touch switch. Therefore, if a foreign article touches with the touch switch inadvertenly, the automatic focusing device inadvertently starts thus, the photographic lens is moved suddenly, surprising the operator. In addition undesired power is consumed. Furthermore, if there is provided with an auxiliary light source in a camera for effecting illumination when focus detection is difficult because of lack of light value as disclosed in Japanese Patent Publication No. 132734/1983, the auxiliary light source is inadvertently illuminated upon touching the shutter release button, surprising the operator and undesired power is consumed.